Transition Metal Dihalides Research Articles

Atomically Precise Vacancy Lattices in Epitaxially Grown FeBr2 and CoBr2 on Au(111)

The generation of extensive 2D periodic patterns of point defects in 2D materials, such as vacancy lattices, has been a challenging task until now. Here, we report on 2D transition metal dihalides grown epitaxially on Au(111) featuring periodically assembled halogen vacancies that result in alternating coordination of the transition metal ion and can function as antidot lattices. [1] Using low-temperature STM/ncAFM and LEED, we identified the structural properties of intrinsically patterned FeBr2 and CoBr2 monolayers grown epitaxially on Au(111). Density-functional theory indicates that Br-vacancies are favored due to low formation energies, and the formation of a vacancy lattice substantially reduces the lattice mismatch with the underlying Au(111). We demonstrate that interfacial strain engineering presents a versatile strategy for controlled patterning in 2D with atomic precision over several hundred nanometers to solve a longstanding challenge of growing atomically precise antidot lattices.[1] https://doi.org/10.48550/arXiv.2305.06489

Intrinsically Patterned Two-Dimensional Transition Metal Halides.

Patterning and defect engineering are key methods for tuning the properties and enabling distinctive functionalities in two-dimensional (2D) materials. However, generating 2D periodic patterns of point defects in 2D materials, such as vacancy lattices that can serve as antidot lattices, has been elusive until now. Herein, we report on 2D transition metal dihalides epitaxially grown on metal surfaces featuring periodically assembled halogen vacancies that result in alternating coordination of the transition metal atom. Using low-temperature scanning probe microscopy and low-energy electron diffraction, we identified the structural properties of intrinsically patterned FeBr2 and CoBr2 monolayers grown epitaxially on Au(111). Density functional theory reveals that Br vacancies are facilitated by low formation energies, and the formation of a vacancy lattice results in a substantial decrease in the lattice mismatch with the underlying Au(111). We demonstrate that interfacial strain engineering presents a versatile strategy for controlled patterning in two dimensions with atomic precision over several hundred nanometers to solve a long-standing challenge of growing atomically precise antidot lattices. In particular, patterning of 2D materials containing transition metals provides a versatile method to achieve unconventional spin textures with noncollinear spin.

Large magnetocaloric refrigeration performance near room temperature in monolayer transition metal dihalides

Magnetocaloric effect (MCE) exhibits highly efficient and ecological cooling abilities for solid-state refrigeration in contrast to traditional vapor-compression refrigeration. Successive emerging two-dimensional (2D) magnetic materials provide a fertile platform for exploring low-dimensional MCE systems. Here, we focus on a series of 2D transition metal dihalides MX2 (M = Fe, Ru, Os; X = Cl, Br) to explore the maximum isothermal magnetic entropy change (−ΔSmagmax) and adiabatic temperature change (ΔTadmax) under external magnetic field. It is found that FeCl2, FeBr2, and RuCl2 have intrinsically sizable −ΔSmagmax, ΔTadmax, and high thermal conductivity near room temperature, demonstrating superior comprehensive refrigeration performance in comparison with other 2D magnets. It is revealed that strong nearest-neighbor ferromagnetic exchange interaction plays a decisive role in −ΔSmagmax, and the high lattice thermal conductivities of FeCl2 and RuCl2 are attributed to the longer phonon lifetime and larger group velocity of low-frequency acoustic branch. Moreover, moderate strain and carriers doping are able to effectively regulate Curie temperature and magnetocrystalline anisotropy energy and correspondingly enhance −ΔSmagmax. The present work provides important insights for the exploration of 2D magnets for magnetocaloric refrigeration near room temperature.

Computational characterization of novel nanostructured materials: A case study of NiCl[formula omitted

A computational approach combining dispersion-corrected density functional theory (DFT) and classical molecular dynamics is employed to characterize the geometrical and thermomechanical properties of a recently proposed 2D transition metal dihalide NiCl2. The characterization is performed using a classical interatomic force field whose parameters are determined and verified through the comparison with the results of DFT calculations. The developed force field is used to study the mechanical response, thermal stability, melting and solidification of a NiCl2 monolayer on the atomistic level of detail. The 2D NiCl2 sheet is found to be thermally stable at temperatures below its melting point of ∼695 K. At higher temperatures, several subsequent structural transformations of NiCl2 are observed, namely a transition into a porous 2D sheet and a 1D nanowire. The MD simulations of NiCl2 cooling show that the molten NiCl2 system solidifies into an amorphous porous 2D structure at T∼450 K. The resulting structure has lower cohesive energy with respect to the initial 2D sheet. The computational methodology presented through the case study of NiCl2 can also be utilized to study the properties of other novel 2D materials, including recently synthesized NiO2, NiS2, and NiSe2.

Magnetic structure and exchange interactions of transition metal dihalide monolayers: First-principles studies

Magnetic structure and exchange interactions of transition metal dihalide monolayers: First-principles studies

Bipolar ferromagnetic semiconductors and dipole-modulated magnetism in two-dimensional Janus transition metal dihalides

Two-dimensional (2D) transition metal dihalides (TMDHs) have attracted great interest owing to their unique magnetic and semiconductor properties. Compared with the mirror/inversion symmetric materials, 2D Janus materials possess vertical intrinsic dipole moment, which offer a versatile platform for the fundamental physics and future spintronic devices. Here, we systematically explore the magnetic and electronic properties of the 2D Janus transition metal dihalides MXY (M = Co and Ni; X ≠ Y = Cl, Br, and I) monolayers and bilayers by using density functional theory. The monolayer CoClBr, NiClBr, and NiBrI are bipolar ferromagnetic semiconductors that possess the valence and conduction band edges of different spin channels. The magnetism of the bilayer CoClBr, NiClBr, and NiBrI is highly dependent on the accumulated dipole moments of the two adjacent layers. When the dipole moments in both layers are aligned in the same direction and the accumulated dipole moments are nonzero, the systems are antiferromagnetic half semiconductors. However, when the dipole moments in the two layers are opposite and the accumulated dipole moments are zero, the systems are A-type antiferromagnetic semiconductors. Our findings are helpful to understand the magnetism of Janus TMDHs and guide experiments in exploring their potential application in spintronic devices.

Manipulating single excess electrons in monolayer transition metal dihalide

Polarons are entities of excess electrons dressed with local response of lattices, whose atomic-scale characterization is essential for understanding the many body physics arising from the electron-lattice entanglement, yet difficult to achieve. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we show the visualization and manipulation of single polarons in monolayer CoCl2, that are grown on HOPG substrate via molecular beam epitaxy. Two types of polarons are identified, both inducing upward local band bending, but exhibiting distinct appearances, lattice occupations and polaronic states. First principles calculations unveil origin of polarons that are stabilized by cooperative electron-electron and electron-phonon interactions. Both types of polarons can be created, moved, erased, and moreover interconverted individually by the STM tip, as driven by tip electric field and inelastic electron tunneling effect. This finding identifies the rich category of polarons in CoCl2 and their feasibility of precise control unprecedently, which can be generalized to other transition metal halides.

Mechanical behavior of monolayer MoS2 films with arrayed dislocation defects

Compared with single and randomly distributed defects, arrayed defects exist widely in the film structure and can be designed artificially to achieve various specific functions. However, the mechanical behavior of transition metal dihalides with dislocation array defects is unclear; this limits their application in new flexible nanodevices. In this study, the strength and fracture properties of monolayer MoS2 films containing 5|7 (S5|7 and Mo5|7) dislocation defects under uniaxial tension were studied by molecular dynamics simulation. The results show that the number of defects, adjacent spacing, and alignment angle affect the critical strain and tensile strength of the monolayer MoS2 films. These results provide a basis for further understanding the mechanical behavior of monolayer MoS2 thin films and provide a theoretical reference for the application of nano-electronic devices based on MoS2.

The Combined Effects of an External Field and Novel Functional Groups on the Structural and Electronic Properties of TMDs/Ti3C2 Heterostructures: A First-Principles Study

The stacking of Ti3C2 with transition metal dihalide (TMDs) materials is an effective strategy to improve the physical properties of a single material, and the tuning of the related properties of these TMDs/Ti3C2 heterostructures is also an important scientific problem. In this work, we systematically investigated the effects of an external field and novel functional groups (S, Se, Cl, Br) on the structural and electronic properties of TMDs/Ti3C2X2 heterostructures. The results revealed that the lattice parameters and interlayer distance of TMDs/Ti3C2 increased with the addition of functional groups. Both tensile and compressive strain obviously increased the interlayer distance of MoS2/Ti3C2X2 (X = S, Se, Cl, Br) and MoSe2/Ti3C2X2 (X = Se, Br). In contrast, the interlayer distance of MoSe2/Ti3C2X2 (X = S, Cl) decreased with increasing compressive strain. Furthermore, the conductivity of TMDs/Ti3C2 increased due to the addition of functional groups (Cl, Br). Strain caused the bandgap of TMDs to narrow, and effectively adjusted the electronic properties of TMDs/Ti3C2X2. At 9% compressive strain, the conductivity of MoSe2/Ti3C2Cl2 increased significantly. Meanwhile, for TMDs/Ti3C2X2, the conduction band edge (CBE) and valence band edge (VBE) at the M and K points changed linearly under an electric field. This study provides valuable insight into the combined effects of an external field and novel functional groups on the related properties of TMDs/Ti3C2X2.

Selenium defects of 2D VSe2/CNTs composite as an efficient sulfur host for high-performance Li-S batteries

Introducing defects into two-dimensional materials can increase the ligand-unsaturated sites, thereby enhancing the redox catalysis of polysulphides. In this study, the selenium defects in the two-dimensional material VSe2 can effectively improve the electrochemical performance of lithium-sulfur batteries. Compared with VSe2, VSe2−x could strongly adsorb polysulfides, catalyze the conversion and accelerate the redox reactions of polysulfides. S/VSe2−x/CNTs cathode with an initial discharge specific capacity of 1457 mAh g−1 at 0.1 C current density and capacity decay rate per cycle of only 0.039% after 800 cycles at 0.5 C, showing excellent cycling stability. In addition, a high areal capacity of 4.35 mAh cm−2 and a reversible capacity of 4.106 mAh cm−2 after 100 cycles at 0.1 C can be achieved with the S/VSe2−x/CNTs cathode with a high areal sulfur loading of 4.5 mg cm−2. The results show that the shuttle effect is effectively suppressed by introducing defects in the two-dimensional transition metal dihalide compounds. Thus, the electrochemical performance of the sulfur electrode for lithium-sulfur batteries is improved. This work is expected to provide a new perspective for the rational design of sulfur host materials for high-performance lithium-sulfur batteries.

Ab initio study on the electromechanical response of Janus transition metal dihalide nanotubes

We study the electronic response of Janus transition metal dihalide (TMH) nanotubes to mechanical deformations using Kohn–Sham density functional theory. Specifically, considering twelve armchair and zigzag Janus TMH nanotubes that are expected to be stable from the phonon analysis of flat monolayer counterparts, we first compute their equilibrium diameters and then determine the variation in bandgap and effective mass of charge carriers with the application of tensile and torsional deformations. We find that the nanotubes undergo a linear and quadratic decrease in bandgap with tensile and shear strain, respectively. In addition, there is a continual increase and decrease in the effective mass of electrons and holes, respectively. We show that for a given strain, the change in bandgap for the armchair nanotubes can be correlated with the transition metal’s in-plane d orbital’s contribution to the projected density of states at the bottom of the conduction band.

A highly efficient room-temperature NO2 gas sensor based on three-dimensional core–shell structured CoS2 bridged Co3O4@MoS2

In recent years, two-dimensional transition metal dihalides have emerged as a subject of growing research interest in the field of gas sensing.

General Synthesis of 2D Magnetic Transition Metal Dihalides via Trihalide Reduction.

Two-dimensional (2D) transition metal dihalides (TMDHs) have been receiving extensive attention due to their diversified magnetic properties and promising applications in spintronics. However, controlled growth of 2D TMDHs remains challenging owing to their extreme sensitivity to atmospheric moisture. Herein, using a home-built nitrogen-filled interconnected glovebox system, a universal chemical vapor deposition synthesis route of high-quality 2D TMDH flakes (1T-FeCl2, FeBr2, VCl2, and VBr2) by reduction of their trihalide counterparts is developed. Representatively, ultrathin (∼8.6 nm) FeCl2 flakes are synthesized on SiO2/Si, while on graphene/Cu foil the thickness can be down to monolayer (1L). Reflective magnetic circular dichroism spectroscopy shows an interlayer antiferromagnetic ordering of FeCl2 with a Neel temperature at ∼17 K. Scanning tunneling microscopy and spectroscopy further identify the atomic-scale structures and band features of 1L and bilayer FeCl2 on graphene/Cu foil.

Electronic properties of 3dtransition metal dihalide monolayers predicted byDFTmethods: Is there a pattern or are the results random?

AbstractWe use periodic DFT calculations at LDA and PBE level to investigate 3dtransition metal dihalide (TMDH) monolayers in H‐ and T‐phase. By analyzing the phonon dispersion, we have obtained a rough overview which combinations may form stable structures. We have focused on identifying and explaining trends in the predicted electronic properties. Although their geometric structures are simple, the associated electronic and magnetic properties are not as easy to understand. At first glance, it seems that there is no clear trend, as even isovalence‐electronic TMDH monolayers formed from the same metal but different halides can feature different magnetic moments. The identification of potential trends is further complicated by the fact that for a significant number of species, LDA results and PBE results predict different ground‐state electronic structures. By rigorously analyzing the potential energy surfaces associated with different magnetic moments, we could show that the apparent inconsistencies can be easily understood as a result of the differences in the relative energy between electronic states of different magnetic moments. We further show that the trends in the band gaps can be easily rationalized by an electron counting rule based on simple symmetry arguments.

Tunable magnetic and optical properties of transition metal dihalides by cation alloying

Alloying has been a tradition in materials science that has enabled groundbreaking discoveries in semiconductor technologies, optics, and photovoltaics, among others. While alloying in traditional systems is relatively well established, the effects of alloying in the emerging van der Waals (vdW) two-dimensional (2D) magnets are still in their infancy. Using ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{Cl}}_{2}$ as a testbed system, our results show that chemical vapor transport of stoichiometric mixtures of $\mathrm{Te}{\mathrm{Cl}}_{4}$, Co, and Ni enables the synthesis of highly crystalline vdW magnetic alloys with excellent control over the Ni concentration ($x$) without any tellurium impurities or phase separation. The method is advantageous compared to binary $\mathrm{Co}{\mathrm{Cl}}_{2}$ and $\mathrm{Ni}{\mathrm{Cl}}_{2}$ precursor mixtures which only produce small-sized crystals with a large compositional variation. Magnetic measurements show that the degree of magnetic anisotropy, Weiss temperature, and N\'eel temperature (${T}_{\mathrm{N}}$) strongly correlate to the Ni concentration, offering a tune-knob to engineer the magnetic behavior of transition metal dihalides. First-principles calculations offer further insights into how the increasing Ni content influences the interlayer and intralayer magnetic couplings and the resulting magnetic response. Overall, our findings provide an important avenue toward metal cation alloying in dihalide 2D vdW magnets and offer means to tune their magnetic behavior on demand.

Evidence for a single-layer van der Waals multiferroic.

Multiferroic materials have attracted wide interest because of their exceptional static1-3 and dynamical4-6 magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order that directly induces ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii-Moriya effect3,7. This intrinsic coupling between the magnetic and dipolar order parameters results in high-strength magnetoelectric effects3,8. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for to enable the harnessing of magnetoelectric coupling in nanoelectronic devices1,9,10. Here we report the discovery of type-II multiferroic order in a single atomic layer of the transition-metal-based van der Waals material NiI2. The multiferroic state of NiI2 is characterized by a proper-screw spin helix with given handedness, which couples to the charge degrees of freedom to produce a chirality-controlled electrical polarization. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Combining birefringence and second-harmonic-generation measurementswith theoretical modelling and simulations, we detect a highly anisotropic electronic state that simultaneously breaks three-fold rotational and inversion symmetry, and supports polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly reveals an ordered magnetic polar state that persists down to the ultrathin limit of monolayer NiI2. These observations establish NiI2 and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit.

Flower-like MoSe2@N-doped carbon sub-nanoclusters regulated by MoO3 hexagonal prism as advanced anode for lithium-ion battery

Carbon-coated transition metal dihalide composite materials have shown favorable performance for lithium ion batteries. Herein, 3D flower-like MoSe2@N-doped carbon sub-nanocluster (MoSe2@NC SNCs) with uniform size ~500 nm was successfully synthesized using the pre-made MoO3 hexagonal prisms as the regulator, Se powder and PDA as raw materials through a simple self-assembly method followed by annealing treatment. The as-synthesized MoSe2@NC SNCs composite was characterized by XRD, XPS, SEM and TEM analyses. When fabricated into coin-cells with the counter of Li metal, the MoSe2@NC SNCs anode exhibited enhanced Li storage properties compared to the larger MoSe2@NC MPs electrode without MoO3 regulator, including better rate performance of 480 mA h/g at current density of 4.0 A/g and cycling stability of 501 mA h/g after 320 cycles at high current density of 2.0 A/g. Therefore, the MoSe2@NC SNCs composite may be one of the promising anode candidate materials for constructing LIBs. Furthermore, the strategy of controlling the size of MoSe2@N-doped carbon by adding its pre-treated transition metal oxides may also guide the controllable synthesis of other similar electrode materials.

One-dimensional transition metal dihalide nanowires as robust bipolar magnetic semiconductors.

One-dimensional (1D) materials with robust ferromagnetic ground states are difficult to achieve but provide a significant platform for potential spintronic device applications in future. Herein, a new family of 1D transition metal dihalide (TMCl2; where TM = Cu, Co, Cr) nanowires are proposed by using first-principles calculations. Their dynamic stability is ensured by Born-Oppenheimer molecular dynamics simulations. The electronic structures demonstrate that both CoCl2 and CuCl2 nanowires are promising bipolar magnetic semiconductors (BMSs) and can be converted into 1D half-metal materials by a small amount of carrier doping. The CrCl2 nanowire is an antiferromagnetic semiconductor (AFS). The formation of a BMS is attributed to the superexchange coupling between the Co/Cu atoms through the 3p orbitals in the Cl atoms. By using Monte Carlo simulations, we found that the CoCl2 nanowire has a Curie point of 6 K, while the CuCl2 nanowire has a corresponding Curie point of 14 K. Our results allow us to put forward a strategy to realize 1D BMSs and to design low-dimensional AF spintronic devices.

High throughput study on magnetic ground states with Hubbard U corrections in transition metal dihalide monolayers.

We present a high throughput study of the magnetic ground states for 90 transition metal dihalide monolayers TMX2 using density functional theory based on a collection of Hubbard U values. Stable geometrical phases between 2H and 1T are first determined. Spin-polarized calculations show that 50 out of 55 magnetic TMX2 monolayers are energetically prone to the 1T phase. Further, the magnetic ground states are determined by considering four local spin models with respect to different U values. Interestingly, 23 out of 55 TMX2 monolayers exhibit robust magnetic ground orderings which will not be changed by the U values. Among them, NiCl2 with a magnetic moment of 2 μB is a ferromagnetic (FM) insulator, while the VX2, MnX2 (X = Cl, Br and I), PtCl2 and CoI2 monolayers have noncollinear antiferromagnetic (120°-AFM) ground states with a tiny in-plane magnetic anisotropic energy, indicating flexible magnetic orientation rotation. The exchange parameters for both robust FM and 120°-AFM systems are analyzed in detail with the Heisenberg model. Our high-throughput calculations give a systematic study of the electronic and magnetic properties of TMX2 monolayers, and these two-dimensional materials with versatile magnetic behavior may have great potential for spintronic applications.

Pressure-induced band-gap closure and metallization in two-dimensional transition metal halide CdI2

In this letter, we present the pressure-induced evolution of the band-gap, conductance, and crystal structure of layered transition metal dihalide (TMH), CdI2, an insulator at ambient conditions, using electrical transport measurement, UV–vis absorption spectroscopy, X-ray diffraction, and Raman scattering spectroscopy. We found that the band gap shrinks gradually following a sharp drop at 34.5 GPa. Meanwhile, the temperature-dependent resistance indicated an insulator-to-semiconductor and then metal transition occurred at 36 and 62 GPa, respectively. Both X-ray diffraction and Raman scattering measurements indicate that the CdI2 underwent a first-order transition from a hexagonal to monoclinic phase at ∼32 GPa due to collapses in the c-lattice parameter and volume. The second structural phase transition (SPT) from monoclinic-to-tetragonal occurred at 48 GPa. The pressure-induced insulator-to-semiconductor-to-metal of CdI2 is attributed to the structural transition from hexagonal-to-monoclinic-to-tetragonal. Our first-principle calculations further confirm the sequence of the SPTs and the semiconducting and metallic band structure of the phases, respectively. Namely, the metallization is observed due to the filled 5p-iodide to shift and overlap with the filled 5s-cadmium band in metal phase. These findings pave the way for investigating crystal structure evolution, and the optical and electrical properties in CdI2-type compounds under extreme conditions.